1. Field of the Invention
The present invention relates to a semiconductor device having a mixture of thin-film semiconductor areas and thick-film semiconductor areas and a manufacturing method for this semiconductor device.
2. Description of the Related Art
In recent years, there has been an increasing demand for an SoC (System on a Chip) that realizes an inexpensive and high-performance system by integrating on the same semiconductor chip a plurality of functional blocks including a high-performance digital circuit such as an MPU (Microprocessor Unit), a semiconductor memory such as a DRAM, and an analog circuit. To develop an SoC providing higher performance, it is important to deal simultaneously with differences in element structures used for the functional blocks and differences in characteristics required for the elements.
The recent technical trend, particularly in the field of digital circuits, is to use MOSFETs having finer elements and reduced parasitic capacitances that have been achieved utilizing an SOI (Silicon On Insulator) structure. It is thus necessary to provide a technique of mixing a DRAM and an analog circuit on an SOI substrate.
However, in a semiconductor memory such as a DRAM or in an analog circuit, a floating body effect of the SOI structure may cause improper circuit operations, for example, malfunctioning and the degradation of a noise characteristic. It is thus difficult to introduce the SOI structure without any modification.
Thus, a method of using a substrate having both an SOI substrate area and a bulk substrate area and forming elements of functional circuits suitable for the respective areas is described in, for example, H. L. Ho et al. at IBM Microelectronics Semiconductor Research and Development Center “A 0.13 μm High-Performance SOI Logic Technology with Embedded DRAM for System-On-Chip Application” IEDM Technical Digest pp. 503 to 506 in 2001.
The method disclosed in this document forms an SOI area by selectively forming a BOX oxide film on a bulk substrate using a SIMOX (Separation by Implanted Oxygen) method.
However, with this method, since the BOX oxide film is formed inside the substrate, a crystal defect may occur at the boundary part between each SIMOX part and a corresponding non-SIMOX part. The height of the surface of the substrate may vary between the SIMOX area and the non-SIMOX area to form steps. This may reduce the yield associated with the formation of fine patterns. Furthermore, for the film configuration of the SIMOX part, i.e. a film composed of the SOI film and the BOX oxide film, the process window is narrow which is used to suppress crystal defects and the creation of pin holes. In particular, it is difficult to form, on the same substrate, areas having different SOI film thicknesses and/or a different BOX oxide film thicknesses.